Diagnosis: atypical fibroxanthoma or not? Evaluating spindle cell malignancies on sun damaged skin: a practical approach.

Poorly differentiated spindle cell malignancies on sun damaged skin frequently pose a diagnostic challenge for dermatopathologists. The vast majority of these neoplasms ultimately are diagnosed as either atypical fibroxanthoma (AFX), spindle cell squamous cell carcinoma (SCSCC), or spindle cell melanoma (SCM), and rarely leiomyosarcoma or angiosarcoma. Light microscopic clues may suggest one of these neoplasms, but subtle and overlapping characteristics often render precise diagnosis impossible based on morphological features alone. Immunohistochemistry therefore is necessary to firmly and accurately diagnose the majority of spindle cell malignancies on sun damaged skin. We summarize typical clinical and histological findings associated with this group of malignancies and offer a practical immunohistochemical approach to use in their diagnosis.